1. Field of the Invention
The present invention relates to a driving device for driving a self-luminous display, such as an EL (electroluminescence) element or an organic EL element, and more particularly to a driving device for constant-voltage driving a self-luminous display element by continuously making on and off states of application of a constant voltage to the self-luminous display element.
2. Description of the Related Art
In recent years, as a self-luminous display, an EL (electroluminescence) element or an organic EL element has been put to practical use. Especially the organic EL element realizes high luminance light emission and high efficiency, it is driven with a DC low voltage, and it has a high speed response. Therefore it is ideal as a light emitting type display. Such a self-luminous display element has a feature that its light emission luminance is in proportion to current density as shown in FIG. 8, and a feature that its V-I characteristics are changed with the elapse of time as shown in FIGS. 9 and 10, that is, it is deteriorated with time so that a current made to flow to obtain luminance of a constant value is changed. As a driving method of the self-luminous display element, there are two kinds of methods, constant-current driving and constant-voltage driving.
As a conventional driving device for driving the self-luminous display element, there is circuit for making constant-current driving. In the case where the self-luminous display element is divided into a plurality of segments or dots, this constant-current driving circuit is provided for the respective dots or segments on a one-to-one basis, and drives the respective segments or dots. According to the constant-current driving circuit, since a constant current is supplied to the self-luminous display element, even if the self-luminous display element is deteriorated and the V-l characteristics are changed, a supplied current is not changed nor the luminance is not changed.
However, since this conventional constant-current driving circuit must be provided for each dot or segment on a one-to-one basis, there has been disadvantages that its device becomes large or complicated, and the cost is increased. Particularly, in the case where the areas of the respective segments are different from each other, since different constant-current driving circuits designed to correspond to the respective areas must be prepared, there has been a disadvantage that its device becomes further complicated and the cost is increased.
As a conventional self-luminous display element driving device for solving the foregoing disadvantages, there is a circuit for making constant-voltage driving. With respect to this constant-voltage driving circuit, only one circuit is provided for a plurality of dots or segments, and applies a constant voltage to the plurality of dots or segments at a constant duty cycle. According to this constant-voltage driving circuit, since only one constant-voltage driving circuit is to be provided for the plurality of dots or segments, its device can be made simple or small, and the cost can be reduced.
However, according to the foregoing conventional constant-voltage driving circuit, since a constant voltage is applied to a self-luminous display element at a constant duty cycle, there has been a problem that in the case where the V-l characteristics of the self-luminous display element is changed with the elapse of time, current flowing to the self-luminous display element is changed by this and the luminance of the self-luminous display element is changed, so that suitable luminance can not be obtained.